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href="/python/secure-shell-connection-in-python.html" class="sidebar-link">Secure Shell Connection in Python</a></li><li><a href="/python/python-anti-patterns.html" class="sidebar-link">Python Anti-Patterns</a></li><li><a href="/python/common-pitfalls.html" class="sidebar-link">Common Pitfalls</a></li><li><a href="/python/hidden-features.html" class="sidebar-link">Hidden Features</a></li><li><a href="/python/date-formatting.html" class="sidebar-link">Date Formatting</a></li><li><a href="/python/ijson.html" class="sidebar-link">ijson</a></li><li><a href="/python/django.html" class="sidebar-link">Django</a></li><li><a href="/python/code-blocks-execution-frames-and-namespaces.html" class="sidebar-link">Code blocks, execution frames, and namespaces</a></li><li><a href="/python/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="abstract-base-classes-abc"><a href="#abstract-base-classes-abc" class="header-anchor">#</a> Abstract Base Classes (abc)</h1> <h2 id="setting-the-abcmeta-metaclass"><a href="#setting-the-abcmeta-metaclass" class="header-anchor">#</a> Setting the ABCMeta metaclass</h2> <p>Abstract classes are classes that are meant to be inherited but avoid implementing specific methods, leaving behind only method signatures that subclasses must implement.</p> <p>Abstract classes are useful for defining and enforcing class abstractions at a high level, similar to the concept of interfaces in typed languages, without the need for method implementation.</p> <p>One conceptual approach to defining an abstract class is to stub out the class methods, and then raise a NotImplementedError if accessed. This prevents children classes from accessing parent methods without overriding them first. Like so:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">Fruit</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">check_ripeness</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">raise</span> NotImplementedError<span class="token punctuation">(</span><span class="token string">"check_ripeness method not implemented!"</span><span class="token punctuation">)</span>
<span class="token keyword">class</span> <span class="token class-name">Apple</span><span class="token punctuation">(</span>Fruit<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">pass</span>
a <span class="token operator">=</span> Apple<span class="token punctuation">(</span><span class="token punctuation">)</span>
a<span class="token punctuation">.</span>check_ripeness<span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment"># raises NotImplementedError</span>
</code></pre></div><p>Creating an abstract class in this way prevents improper usage of methods that are not overriden, and certainly encourages methods to be defined in child classes, but it does not enforce their definition. With the <code>abc</code> module we can prevent child classes from being instantiated when they fail to override abstract class methods of their parents and ancestors:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">from</span> abc <span class="token keyword">import</span> ABCMeta
<span class="token keyword">class</span> <span class="token class-name">AbstractClass</span><span class="token punctuation">(</span><span class="token builtin">object</span><span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token comment"># the metaclass attribute must always be set as a class variable </span>
__metaclass__ <span class="token operator">=</span> ABCMeta
<span class="token comment"># the abstractmethod decorator registers this method as undefined</span>
<span class="token decorator annotation punctuation">@abstractmethod</span>
<span class="token keyword">def</span> <span class="token function">virtual_method_subclasses_must_define</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token comment"># Can be left completely blank, or a base implementation can be provided</span>
<span class="token comment"># Note that ordinarily a blank interpretation implicitly returns `None`, </span>
<span class="token comment"># but by registering, this behaviour is no longer enforced.</span>
</code></pre></div><p>It is now possible to simply subclass and override:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">Subclass</span><span class="token punctuation">(</span>AbstractClass<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">virtual_method_subclasses_must_define</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span>
</code></pre></div><h2 id="why-how-to-use-abcmeta-and-abstractmethod"><a href="#why-how-to-use-abcmeta-and-abstractmethod" class="header-anchor">#</a> Why/How to use ABCMeta and @abstractmethod</h2> <p>Abstract base classes (ABCs) enforce what derived classes implement particular methods from the base class.</p> <p>To understand how this works and why we should use it, let's take a look at an example that Van Rossum would enjoy. Let's say we have a Base class "MontyPython" with two methods (joke & punchline) that must be implemented by all derived classes.</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">MontyPython</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">joke</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">raise</span> NotImplementedError<span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token keyword">def</span> <span class="token function">punchline</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">raise</span> NotImplementedError<span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token keyword">class</span> <span class="token class-name">ArgumentClinic</span><span class="token punctuation">(</span>MontyPython<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">joke</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span> <span class="token string">"Hahahahahah"</span>
</code></pre></div><p>When we instantiate an object and call it's two methods, we'll get an error (as expected) with the <code>punchline()</code> method.</p> <div class="language- extra-class"><pre class="language-text"><code>
>>> sketch = ArgumentClinic()
>>> sketch.punchline()
NotImplementedError
</code></pre></div><p>However, this still allows us to instantiate an object of the ArgumentClinic class without getting an error. In fact we don't get an error until we look for the punchline().</p> <p>This is avoided by using the Abstract Base Class (ABC) module. Let's see how this works with the same example:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">from</span> abc <span class="token keyword">import</span> ABCMeta<span class="token punctuation">,</span> abstractmethod
<span class="token keyword">class</span> <span class="token class-name">MontyPython</span><span class="token punctuation">(</span>metaclass<span class="token operator">=</span>ABCMeta<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token decorator annotation punctuation">@abstractmethod</span>
<span class="token keyword">def</span> <span class="token function">joke</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">pass</span>
<span class="token decorator annotation punctuation">@abstractmethod</span>
<span class="token keyword">def</span> <span class="token function">punchline</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">pass</span>
<span class="token keyword">class</span> <span class="token class-name">ArgumentClinic</span><span class="token punctuation">(</span>MontyPython<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">joke</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span> <span class="token string">"Hahahahahah"</span>
</code></pre></div><p>This time when we try to instantiate an object from the incomplete class, we immediately get a TypeError!</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token operator">>></span><span class="token operator">></span> c <span class="token operator">=</span> ArgumentClinic<span class="token punctuation">(</span><span class="token punctuation">)</span>
TypeError<span class="token punctuation">:</span>
<span class="token string">"Can't instantiate abstract class ArgumentClinic with abstract methods punchline"</span>
</code></pre></div><p>In this case, it's easy to complete the class to avoid any TypeErrors:</p> <div class="language-py extra-class"><pre class="language-py"><code><span class="token keyword">class</span> <span class="token class-name">ArgumentClinic</span><span class="token punctuation">(</span>MontyPython<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">def</span> <span class="token function">joke</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span> <span class="token string">"Hahahahahah"</span>
<span class="token keyword">def</span> <span class="token function">punchline</span><span class="token punctuation">(</span>self<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token keyword">return</span> <span class="token string">"Send in the constable!"</span>
</code></pre></div><p>This time when you instantiate an object it works!</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/python/abstract-base-classes-abc.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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